For purposes of diagnosis and treatment planning, imaging techniques are commonly used in medical procedures to view the internal anatomy of a patient's body. In one imaging technique, an imaging catheter with a rotatable ultrasound transducer mounted on its tip is inserted into the patient's body, e.g., through a blood vessel. To obtain an interior image of the body, the rotating ultrasound transducer emits pulses of ultrasound energy into the body. A portion of the ultrasound energy is reflected off of the internal anatomy of the body back to the transducer. The reflected ultrasound energy (echo) impinging on the transducer produces an electrical signal, which is used to form a 360 degree cross-sectional interior image of the body. The rotating ultrasound transducer can be longitudinally translated, so that multiple cross-sectional images can be generated and later reconstructed into a three-dimensional interior image of the body.
Oftentimes, it is desirable to properly orient an image generated by the imaging catheter relative to an anatomical structure (such as, e.g., a heart) or a reference point (such as, e.g., the anterior of a patient). Recently, it has become desirable to properly orient an ultrasonically generated local image of body tissue within a global image of a body or organ containing such body tissue. In order to assist physicians in maneuvering medical devices to sites of interest in the body, such global images are typically generated using a guidance system.
In one guidance system, a fluoroscopic image of the device (or at least radiopaque bands located on the device) and surrounding anatomical landmarks (with or without the use of contrast media) in the body are taken and displayed to the physician. The fluoroscopic image enables the physician to ascertain the position of the device within the body and maneuver the device to the site of interest. In another guidance system using anatomic mapping, a graphical representation of the device or portion of the device is displayed in a three-dimensional computer-generated representation of a body tissue, e.g., a heart chamber. The three-dimensional representation of the body tissue is produced by mapping the geometry of the inner surface of the body tissue in a three-dimensional coordinate system, e.g., by moving a mapping device to multiple points on the body tissue. The position of the device to be guided within the body tissue is determined by placing one or more location sensors on the device and tracking the position of these sensors within the three-dimensional coordinate system. An example of this type of guidance system is the Realtime Position Management™ (RPM) tracking system developed commercially by Cardiac Pathways Corporation, now part of Boston Scientific Corp. The RPM system is currently used in the treatment of cardiac arrhythmia to define cardiac anatomy, map cardiac electrical activity, and guide an ablation catheter to a treatment site in a patient's heart.
In order to properly display the local image within the global image (however generated), both the local image and the global image are registered in a three-dimensional coordinate system. If the global image is a three-dimensional computer-generated representation of the body tissue, it is typically already registered within a three-dimensional coordinate system. Registration of the local image within the three-dimensional coordinate system can be accomplished by mounting a location sensor on the imaging catheter a known distance from the rotating ultrasound transducer, so that the three-dimensional coordinates of the ultrasound transducer, and thus, the origin of the local image can be determined. Depending on the type of location sensor, up to five degrees of freedom (x, y, z, pitch, and yaw) can be determined for the local image.
For example, a plurality of ultrasound sensors, such as those disclosed in U.S. patent application Ser. No. 09/128,304 to Willis et al. entitled “A dynamically alterable three-dimensional graphical model of a body region,” can be mounted along the distal end of the imaging catheter. The geometry of the distal end of the imaging catheter can be extrapolated from the determined positional coordinates of the ultrasound transducers, so that the three positional coordinates (x, y, z) and two rotational coordinates (pitch and yaw) of the imaging element can be determined.
As another example, a magnetic sensor, such as those disclosed in U.S. Pat. No. 5,391,199 to Ben-Haim, entitled “Apparatus and Method for Treating Cardiac Arrhythmias,” can be mounted at the distal end of the imaging catheter. Theoretically, these magnetic sensors can be used to determine six degrees of freedom, including roll. Because the roll of a rotating imaging element relative to the distal end of the imaging catheter is not known, however, the roll of the imaging element within the three-dimensional coordinate system cannot currently be determined using the magnetic sensors alone. It would be theoretically possible to mount the magnetic sensor on the rotating shaft to determine the roll of the rotating imaging element. Because these magnetic sensors are relatively large, however, such an arrangement is typically not practical.
As a result, it may be difficult to properly orient an image generated by a rotating imaging element. There thus remains a need for an improved system and method for properly orienting such an image.